The present invention pertains to the radio frequency propagation art and, more particularly, to a directive antenna system for radio frequency signals.
Directive antenna systems are well known, especially in the high frequency communication art. There, it has been found that antenna systems may be arranged such that the radiation pattern from the antenna array may be made to beam in a selected direction.
A common form for such directive antenna systems is the Yagi array. In the Yagi array the signal to be transmitted is fed to a vertical antenna which has a length selected to optimally radiate those signals at the frequency of the signal to be transmitted. Thus, the antenna may be either 1/4 or 1/2 the wave length of the transmitted signal. Placed about the central vertical antenna are a series of parasitic elements which are arranged to either direct or reflect the signal radiated from the central antenna. A directive element is generally comprised of a vertically standing antenna element which has an electrical length less than 1/2 wavelength of the signal being transmitted. A reflective antenna element is one having an electrical length greater than 1/2 the wavelength of the signal to be transmitted. The electrical length of an antenna element may be varied by either coupling a reactive component to the element, or by physically changing the length of the element.
A primary problem with prior art parasitic type antenna arrays is that they require a plurality of vertically standing towers, which, in toto, cover a substantial land surface area. In addition, since a means must be provided to vary the electrical length of each tower, conventional directive antenna arrays have utilized a large "spiderweb" of intercoupling cabling. Moreover, conventional directive antenna arrays have required seven independently standing elements to accomplish the desired beaming, with each element being electrically insulated from ground, thus requiring an intricate grounding system. These requirements have resulted in expensive and costly antenna support structures.